The testing of the functional systems of automobiles, trucks and the like has progressed to a point that extremely sophisticated and detailed tests may be performed to insure both that the components of the vehicle are working properly from a mechanical and electromechanical point of view, and that system performance is in accordance with mandated guidelines, whether they be on the federal, state or local level. The federal Environmental Protection Administration (EPA), for example, has promulgated extensive regulations limiting the emissions of motor vehicles. The present technology allows a battery of tests to be performed by a test technician utilizing a computer controlled interface and analysis system which can provide essentially real time evaluation the vehicle parameters and components under test.
One area of particular concern is the vehicle fuel system. The loss of fuel through evaporation to the atmosphere from the fuel tank and associated piping is both wasteful, as well as being environmentally unsound, as the evaporated fuel, in addition to creating a possibly dangerous situation, contributes to unwanted hydrocarbon pollution. In an effort to curb such pollution and identify vehicles having defective fuel systems which can allow for the loss of fuel, the EPA has proposed a pressure test to determine fuel tank integrity. The test consists of pressurizing the tank and associated vapor lines leading to the vapor capture canister to determine if they are capable of holding pressure and thus are free of leaks. The test is performed by sealing the vent line from the fuel tank at the charcoal canister and pressurizing the tank and lines with nitrogen. The rate of pressure loss over two minutes is then measured.
This test protocol, while able to qualitatively identify the integrity of the fuel system, provides little in the way of a quantitative analysis. It is accordingly the purpose of the present invention to provide a pressure test procedure which addresses the problem of variation in the volume of the evaporative system being tested. When a tank is pressurized to a given pressure, the rate of leak is proportioned to the volume of gas being vented. The difference in volume in the available vapor space in a full and empty gas tank may exceed a factor of 10. Thus, the same size leak can give a ten times faster pressure decay with a full tank as with an empty tank. The present invention allows the free volume of the tank to be considered and allows a quantitative determination of the size of leak, a determination not addressed by the present procedure.